The majority of corrugated boxes, paper grocery bags, fine papers, and market pulps are produced by a sulfate pulping process known as "Kraft" pulping. The process is characterized by the fact that sodium sulfide is added to the medium that is used to cook the wood chips and produce pulp. When this technique was introduced over a century ago, the addition of sodium sulfide produced a dramatic improvement in pulp strength, pulp yield, and durability of the paper made therefrom.
In the typical Kraft digestion process, wood chips are added to an aqueous medium consisting mostly of white liquor which will be transformed into black liquor during the cook. In general, the liquor in which the wood chips are cooked, or cooking liquor, comprises a mixture of black and white liquor, the black liquor being liquor added back to the cooking vessel, or digester, from a prior batch of wood chips and the white liquor being a freshly prepared alkaline solution as described below. Black liquor varies considerably among different mills depending on the white liquor used, the wood employed, and the method of cooking. Typical white liquor is a solution of sodium hydroxide, sodium carbonate, sodium sulfate, sodium sulfide and various inorganic materials. White liquor solubilizes the pulp and removes the lignin from the wood fibers as described below.
The largest part of the organic matter removed from the wood during cooking is combined chemically with sodium hydroxide in the form of sodium salts. Some of these compounds are resin soaps which account for the intense foaming properties of black liquor. In addition, organic sulfur compounds and mercaptans, which give the characteristic odor to the sulfate-containing black liquor, and small amounts of sodium sulfate, silica and other impurities such as lime, oxide, alumina, potash, and sodium chloride are present in the black liquor.
In the pulping process, pre-sized wood chips are subjected to the alkaline reagents at elevated temperatures and pressures in a digester vessel. Generally, temperatures range from about 250.degree. F. to about 350.degree. F. and pressures range from about 60 psi/g to about 130 psi/g. Digestion time may range from 30 minutes to 10 hours, depending on the process conditions and the desired pulp/paper characteristics.
The reaction conditions present during the cook, or digestion, cause lignin--the amorphous polymeric binder found in wood fibers--to be hydrolyzed. Ideally, wood chips are digested only long enough to dissolve sufficient lignin to free the cellulosic wood fibers but maintain sufficient lignin intact to provide added strength to the paper. The pulping process attempts to maximize pulp yield, which is defined as the dry weight of pulp produced per unit dry weight of wood consumed.
After sufficient lignin has been dissolved to free the cellulosic wood fibers, the digester charge is blown into a receiving vessel, or blow tank. The sudden drop in pressure from the digester to the blow tank causes additional mechanical breakup of the wood fibers. In some papermaking applications, the residual lignin is removed to produce papers without the characteristic brown color of Kraft paper. In producing linerboard or Kraft paper, however, the lignin residue remains in the papermaking pulp so that the highest possible strength of wood pulp is achieved.
Ideally, each of the wood chips blown from the digester into the blow tank is broken down into separate wood fibers. In practice, however, some of the wood chips fail to completely separate due, in part, to the undissolved lignin remaining in the pulp. These unseparated particles are removed from the wood pulp by passing the pulp through a screen having openings of a predetermined size. In the pulping industry, the standard test screen employed is flat with 0.01 inch slots therethrough.
The materials that are recovered by this screening process are known as "rejects". The rejects include wood fibers that could be used to produce paper. Accordingly, it is highly desirable to decrease the amount of rejects. One method of lowering the amount of rejects is by increasing the digestion time or by creating more severe hydrolysis conditions. Such conditions, however, increase the costs involved and cause some of the cellulose in the wood chips to be hydrolyzed and rendered unusable.
After contact with liquor in the digester, inorganics, any unused surfactants that may have been added, and solubilized lignin and resins are removed from the pulp in one or more washing steps. Temperatures in the digestion and washing stages typically vary from about 250.degree. F. to 340.degree. F. and 100.degree. F. to 200.degree. F., respectively. After washing, the pulp may be subjected to further bleaching or purification treatments as desired before being sheeted and dried, or prepared for sale, or further utilized in making paper.
A Kappa number corresponds directly to the amount of lignin remaining in the pulp. Generally, the higher the Kappa number, the more lignin present in the pulp and, therefore, the higher the pulp yield. The Kappa number generally decreases as the digestion time is increased or the alkalinity of the cooking liquor is increased. The goal in such Kraft papermaking processes is to retain as much lignin as possible in order to enhance strength and to reduce the cost, while maintaining the uniformity of the cook. More uniform cooks result in a decreased percentage of rejects and, thereby, reduce costs for running paper mills.
Cooking, or digestion, of the pulp may be terminated when the amount of rejects in the pulp is reduced to an acceptable level. Substantial yield and quality advantages are achieved if the wood chips are cooked to a higher lignin content. As a result, an increase in a Kappa number target by the use of thinner chips can result in a substantial cost savings. However, the thickness of chips obtainable on a commercial scale is always variable. A major portion of the total rejects frequently originate from a relatively small fraction of the chips having the greatest thickness. The objective in every pulping process is to achieve a lower percentage of rejects.
In recent years, various surfactants have been added to the pulp cooking medium to increase deresination of the wood pulp. Deresination removes various resins found in wood, including lignin, tannins, and organic solvent-extractable materials, such as fats, fatty acids, resin acids, sterols and hydrocarbons. U.S. Pat. No. 4,426,254 to Wood et al. describes a C.sub.12 -alpha olefin sulfonate or C.sub.21 -dicarboxylic acid as a solubilizing agent in combination with a deresination agent consisting of sodium hydroxide and an ethylene oxide condensation product. The composition removes resins so that fouling of process equipment and foaming in process streams are reduced. Moreover, deresination provides for production of high grade cellulose which may be used in various manufactured cellulose-containing products. Another deresination agent is described in U.S. Pat. No. 2,999,045 to Mitchell et al. as a block copolymer of polyethylene oxide and polypropylene oxide. Such block copolymers as described therein are manufactured and sold under the names PLURONIC L-44, PLURONIC L-62, PLURONIC L-64 and PLURONIC F-68.
A process for enhancing the cooking of wood chips for producing pulp is described in U.S. Pat. No. 4,906,331 to Blackstone et al. As described therein, a block copolymer of polyethylene oxide and polypropylene oxide having a molecular weight of from 500 to 30,000 is added to the pulp cooking liquor to form a Kraft pulp. The polyethylene oxide portion of the block polymer described therein is present in the reagent in an amount of from about 20% to about 80%. Such surfactants are sold by BASF Wyandotte Corporation (hereinafter "BASF") under various tradenames including PLURONIC L-62, PLURONIC L-92 and PLURONIC F-108.
The particular block copolymer surfactants described in the '331 patent have been found to be only partially soluble in both highly alkaline solutions such as white liquor and in low alkaline solutions such as weak black liquor having alkali concentrations as low as 5 grams per liter. Lab work has also shown that a waxy precipitate often forms on the surface of hot white liquor when the surfactant described by the '331 patent is employed.
U.S. Pat. No. 4,952,277 to Chen et al, describes a process for making paper and linerboard employing a phenoxy ethyleneoxy alcohol surface active agent. The particular agent described therein is sold under various names such as IGEPAL.RTM. RC-520, TRITON.RTM. X-100, and SURFONIC.RTM. N-95 sold by GAF Corp., Rohm and Haas Co. and Texaco Chemical Co., respectively. The patent discloses that the surface active agent may be used in combination with the ethylene/propylene block copolymer described in the '331 patent.
Anthraquinone is another reducing agent that has been used as an alternate to sodium sulfide in the Kraft pulping process. The expense of anthraquinone prevents its use by most paper mills. Some of the previously mentioned surfactants, including the block copolymers, have, however, produced a synergistic effect when employed in combination with anthraquinone.
Although various agents and processes have been employed to enhance the cooking of wood pulp as well as to cause deresination, the particular features of the present invention have not heretofore been known. The prior art is generally deficient in affording the stable and soluble composition and process described herein for producing a reduction in pulp rejects and an increase pulp yield. The present invention overcomes the shortcomings of the prior art in that the composition and process disclosed herein result in lower processing costs, easier operational procedures, and increased yield of pulp recovered from various wood sources.